Watermark information (denoted WM) consists of several symbols which are embedded continuously in the carrier content, e.g. in (encoded) audio or video signals, e.g. in order to identify the author of these signals. At decoder site the WM is regained, for example by using correlation of the received signal with a known m-sequence if spread spectrum is used as underlying technology. Most WM technologies transmit redundancy bits for error correction.
In many audio watermarking systems the payload data is organised in frames. A frame starts with one or more synchronisation symbols followed by one or more payload symbols. The synchronisation symbols signal only the start of the payload bits, whereas the payload symbols carry the actual payload bits including the bits used for error correction. The upper part of FIG. 3 shows three successive frames FRn−1, FRn and FRn+1. A frame consists of a number of synchronisation blocks SYNBL (at least one synchronisation block) which are used to detect the start of the frame at decoder side, and a number of payload blocks PLBL (at least one valid payload block or symbol) which carry the actual information. Frames are inserted synchronously or asynchronously into the audio stream, dependent on the technology. The insertion of the payload blocks is done consecutively, i.e. synchronised after the SYNBL blocks. Each payload block holds one or more bits of information.
Many audio watermarking technologies like spread spectrum, or phase shaping disclosed in EP05090261, embed some kind of reference sequences in the carrier signal. If binary phase keying (BPSK) is used, the polarity of the sequence encodes the bit value. For code shift keying (CSK), different sequences are used for the different values of the transmitted bit value. The lower part of FIG. 3 shows a frame that starts with three synchronisation symbols S1, S2, and S3 which are followed by eight payload symbols Pld1 to Pld8. At detector or receiver side it happens that a received erroneous watermark symbol cannot be decoded for example because of attacks. The payload data is then error corrected and decoded.